1. Field of the Invention
This invention relates to an improved process for extraction of anions, particularly complex anions such as auricyanide, from aqueous solutions employing quaternary amine extraction reagents in either liquid-liquid or liquid-solid systems.
2. Description of Related Art
Clarified leach liquors containing the gold are obtained by leaching with cyanide solutions through either the dump or heap leaching techniques. In heap leaching, the ore is placed on specially prepared impervious pads and a leaching solution is then applied to the top of the heap and then allowed to percolate down through the heap. The solution containing the dissolved metal values eventually collects along the impervious pad and flows along it to a collection basin. From the collection basin, the solution is pumped to the recovery plant. Dump leaching is similar to heap leaching. Old mine waste dumps which have sufficient metal value to justify processing are leached in place. The gold in clarified leach solutions may be recovered by direct precipitation in the Merrill-Crowe process, or by adsorption on charcoal in columns (CIC), followed by either electrowinning or by precipitation in the Merrill-Crowe process.
In certain conditions, unclarified solutions are generated by agitated vat leaching. In this continuous Carbon in Pulp (CIP) leaching process, the ore is slurried with agitated leach solution in the presence of carbon granules to generate a pulp. Dissolved gold is adsorbed onto the carbon resulting in low aqueous gold concentrations, which often increases the rate and completeness of gold extraction from the ore. Carbon granules carrying the gold are separated from the pulp by screening, and the gold is recovered from the carbon typically by elution with sodium hydroxide solution, which may contain some free cyanide, followed by electrowinning. Before the carbon granules can be returned to the leaching step, they must be reactivated by hazardous and expensive washing and heating steps. Coconut shell activated carbon is preferred, but is in short supply and expensive.
That one can effectively extract auricyanide from alkaline cyanide leach liquors with quaternary amines either as solvent extraction reagents or as functionality mounted on a resin is well known. The use of the quaternary amine functionality for the recovery of gold in the mining industry is limited, however, by the fact that recovery of the auricyanide away from the quaternary amine is very difficult.
Stripping of the auricyanide away from the quaternary amine is currently accomplished by multi-step processes involving acidification and generation of HCN gas at some stage. As a result, these processes involve substantial cost and risk due to the necessity of recovering and recycling HCN. One process involves contacting the loaded extractant (solution or resin) with an aqueous acidic solution of thiourea. The auricyanide anion is converted to a cationic gold thiourea complex which no longer is attracted to the quaternary amine, and the cyanide is released as HCN gas, which is very toxic. Due to the cost of cyanide, it is necessary to recover the HCN by neutralization with caustic. An alternate approach, which is finding some use, involves displacing the auricyanide from the loaded extractant by contact with an alkaline tetracyanozinc solution. The tetracyanozinc anion is preferentially extracted by the quaternary amine and displaces the auricyanide from the extractant phase. Removal of the tetracyanozinc species from the extractant then requires contacting with an aqueous sulfuric acid solution with resultant HCN generation.
Alternate ways which have also been suggested as possible routes for recovering auricyanide away from the quaternary amine include burning the loaded extractant. This approach suffers from the fact that one destroys the extractant, and it is uneconomic except in cases such as waste treatment where no mining costs are incurred, or where a very high level of loading has been achieved.
More specifically in regard to extraction of anions, such as auricyanide using amine, may be mentioned Canadian Patent 587,403 to Kember et al., in which the recovery of gold from aqueous cyanide gold bearing solutions using ion exchange resins having at the majority of positions primary, secondary or tertiary amine groups but containing up to 25% quaternary ammonium groups.
Miller and Mooiman in U.S. Pat. No. 4,774,003 and their published article, "Selectivity Consideration in the Amine Extraction of Gold from Alkaline Cyanide Solutions, Minerals and Metallurgical Processing", August, 1984, pp. 153-157, describe the ion exchange extraction of metallic and non-metallic anions by control of the basicity of primary, secondary and tertiary amines with Lewis bases, such as tributylphosphate. The publication notes on page 153 that quaternary amines have been studied for extraction of gold but that while extraction was possible, stripping was difficult.
Burstall, F. H. et al. in "Ion Exchange Process for Recovery of Gold from Cyanide Solution", Industrial and Engineering Chemistry, Vol. 45, No., 8, August, 1953, pp. 1648-1658, describe an investigation into the absorption of gold from cyanide solutions using a strongly basic resin Amberlite IRA-400.
More recently, guanidine reagents have been proposed for extraction of precious metals, such as gold, from aqueous, alkaline, cyanide solutions, notably in Henkel Corporation, U.S. Pat. Nos. 4,814,007 to Wilson Lin et al. Other patents utilizing guanidine compounds are South African Patent 71/4981 for extraction of gold from aqueous acidic solutions and South African Patent 89/2733 describing a similar process for recovering gold from alkaline cyanide solutions.
In U.S. Pat. No. 4,661,257, quaternary ammonium salts are described for use in ion transport processes which employ supported liquid membranes. In this process the quaternary amine cation forms an ion pair with a target anion, such as nitrate and a counterion, such as phenolate, is used for a high affinity for hydrogen ion.